Platelet-derived growth factor (PDGF) has been proposed to be one of the growth factors which drive proliferation during normal development and in various pathological conditions, including vascular response to injury, hypertension, and atherosclerosis. Support for these hypotheses has been largely circumstantial, usually consisting of an observed correlation between proliferation and expression of PDGF and PDGF receptors. We propose to develop "chimeric" mice as tools for directly evaluating the role of components of the PDGF/PDGF receptor system in vivo. Chimeric mice are created by combining early embryonic cells from two different embryos. Our first goal will be to create chimeras in which both components are normal but one is marked in such a way that derivatives of the two components can be distinguished unambiguously. These chimeras will be used to obtain basic information about the way in which normal cells proliferate and migrate during the development of the vessel wall and during pathological changes. We will then prepare chimeras between marked normal and mutant cells and use these to investigate the consequences of elimination of components of the PDGF/PDGF receptor system. We will begin by investigating the role of the PDGF receptor alpha-subunit (PDGFRalpha) using the Patch mutant. We have shown that the Patch mutation deletes the PDGFRalpha gene. We will use cell lines that we have derived from Patch mutant embryos to create chimeric mouse embryos in which cells without the PDGFRalpha gene can survive and be distinguished histologically (using the marker) from cells with the PDGFRalpha gene. By comparing the behavior of adjacent receptor-positive and receptor-negative cells, we will be able to directly evaluate the role that PDGFRalpha plays in the development of the vascular system, and in the response of vascular cells to pathological perturbations, including injury, hypertension, and atherogenesis. As time permits, the techniques and reagents developed for analysis of PDGFRalpha will be used to evaluate the role of other components of the PDGF/PDGF receptor system, beginning with the PDGF receptor beta-subunit (PDGFRbeta). Based on the pattern of expression of the two PDGF receptor proteins, we anticipate that the PDGFRbeta mutants will have a lesser role during development and a greater role in mediating cell responses in adult pathological conditions.